7.3 Detailed Description of diff3 Normal Format

Each hunk begins with a line marked ‘====’. Three-way hunks have
plain ‘====’ lines, and two-way hunks have ‘1’, ‘2’, or
‘3’ appended to specify which of the three input files differ in
that hunk. The hunks contain copies of two or three sets of input
lines each preceded by one or two commands identifying where the lines
came from.

Normally, two spaces precede each copy of an input line to distinguish
it from the commands. But with the --initial-tab (-T)
option, diff3 uses a tab instead of two spaces; this lines up
tabs correctly. See Tabs, for more information.

Commands take the following forms:

‘file:la’

This hunk appears after line l of file file, and
contains no lines in that file. To edit this file to yield the other
files, one must append hunk lines taken from the other files. For
example, ‘1:11a’ means that the hunk follows line 11 in the first
file and contains no lines from that file.

‘file:rc’

This hunk contains the lines in the range r of file file.
The range r is a comma-separated pair of line numbers, or just one
number if there is only one line. To edit this file to yield the
other files, one must change the specified lines to be the lines taken
from the other files. For example, ‘2:11,13c’ means that the hunk
contains lines 11 through 13 from the second file.

If the last line in a set of input lines is incomplete
(see Incomplete Lines), it is distinguished on output from a full
line by a following line that starts with ‘\’.